The crystal structure of titanium oxide has three types that are a rutile type of a tetragonal high-temperature type, an anatase type of a tetragonal low-temperature type, and an orthorhombic brookite type, and the anatase-type titanium oxide is known to have high photocatalytic action and high photohydrophilic action.
For the photocatalytic action of titanium oxide, it is known that in the surface of titanium oxide, active oxygen such as O2− (super oxide ion) and —OH (hydroxyl radical) is generated by a photoexcitation with ultraviolet light or the like, so that an action of decomposing organic substances is developed. The photohydrophilic action of titanium oxide is known to be developed by such a mechanism that a hydroxy group is generated in the surface of titanium oxide by a photoexcitation with ultraviolet light or the like, so that the contact angle of titanium oxide relative to water is largely lowered (to 20° or less).
The anatase-type titanium oxide is known to have the highest photocatalytic action and the highest photohydrophilic action among the above three crystal types of titanium oxide and is used in various applications such as antimicrobial, antifouling, deodorizing, odor eliminating, anti-fogging, and hydrophilization.
For more effectively developing the photocatalytic action and photohydrophilic action, titanium oxide having a large specific surface area is advantageous and a titanium oxide sol in which primary particles are fine and are in a homogeneous dispersion state is preferred.
As a production method of an anatase-type titanium oxide sol, there is disclosed a method including reacting a water-soluble titanium compound with ammonia to generate a gel, subjecting the gel to a hydrothermal treatment at 100° C. or more, and adding an acid to the gel (see Patent Document 1). There is also disclosed a method for obtaining a sol by bringing an anatase-type titanium oxide powder in an aqueous phase into contact with either a cation exchanger in the presence of an acid or with an anion exchanger in the presence of an alkali (see Patent Document 2).
As a production method of an anatase-type titanium oxide sol using a titanium alkoxide as a titanium source, there is disclosed a method including adding diethylene glycol as a complexing agent to titanium isopropoxide, distilling off an alcohol from the resultant reaction mixture, adding hydrochloric acid and acetic acid together with water to the reaction mixture, and subjecting the reaction mixture to a reaction under reflux at 180° C. (see Patent Document 3).
There is also disclosed a method including mixing a titanium oxychloride aqueous solution with citric acid, adding ammonia to the resultant reaction mixture, and heating the reaction mixture to 90° C. to obtain a titanium oxide sol (see Patent Document 4).